These days, semiconductor devices have been highly integrated. Thus, highly-flat and high quality silicon wafers are required. In case of polishing interlayer insulating films and metal wires deposited on a surface of a wafer to produce semiconductor devices, the surface of the wafer must be more highly flattened. Therefore, a polishing apparatus capable of more accurately polishing a surface of a wafer as a mirror surface or highly precisely polishing the surface as a standard surface is required.
For example, a conventional wafer polishing apparatus has an airbag-type polishing head, which holds a wafer and presses the entire surface of the wafer onto a polishing pad of a polishing plate with uniform pressing force (see Japanese Laid-open Patent Publication No. 2002-187060).
According to claim 1 of said Japanese laid-open patent publication, the wafer polishing apparatus has a polishing head comprising: a top ring having a main body for holding a wafer; an elastic pad contacting the wafer; and a supporting member for supporting the elastic pad. A contact member having an elastic sheet, which contacts the elastic pad, is provided on a lower face of the supporting member. A first pressure chamber, which is located in an inner part of the contact member, and a second pressure chamber, which is located in an outer part of the contact member, are formed in a space between the elastic pad and the supporting member. A fluid supply source supplies a fluid to or vacuates the first and second pressure chambers.
In the polishing apparatus disclosed in said Japanese laid-open patent publication, the wafer is concentrically pressed by the first and second pressure chambers and the elastic pad. Therefore, the wafer can be uniformly polished by adjusting inner pressures of the first and second pressure chambers.
However, the inner pressures of the first and second pressure chambers are different, so pressure in a boundary area between the first and second pressure chambers must be discontinuous. Therefore, in the boundary area, the pressure distributes in a staircase pattern, and the surface of the wafer will be polished like a staircase.
In case of using the elastic pad composed of a softer material, the above described problem occurs remarkably.